This invention relates in general to battery chargers, and more specifically to a method and apparatus for charging a battery in a portable communication device from a variety of power sources, including limited capacity sources such as an integral power node of a computer data bus. One such computer data bus would be a USB (universal serial bus) port.
With the current computing and information revolution, portable electronic devices such as cellular telephones, personal digital assistants (PDAs), digital pagers and wireless email devices, are becoming very common. These portable devices are typically powered by internal batteries which must be recharged periodically by an external power source, using a battery charger. Battery chargers generally receive power from a standard AC electrical outlet and convert the AC power into a low DC voltage for recharging a battery.
The battery chargers of these portable devices also generally employ a “battery charge controller” to manage the charging of the battery. Such battery charge controllers offer functionality such as:                regulating the voltage and current levels to the rechargeable battery;        providing status signals to the main processor of the portable device, or operating one or more status LEDs (light emitting diodes);        providing protection circuits such as overcurrent, undervoltage, reverse polarity and overtemperature protection; and        shutting themselves off when the charging source has been removed, to minimize battery drain.        
Lithium ion battery packs, for example, must be charged according to a relatively strict algorithm so that they can be fully charged, be charged and re-charged many times, and be safely operated. This charging algorithm generally proceeds as follows:                1. at the first stage, any severe undervoltage or deep discharge conditions are addressed. During this pre-charging stage, the battery voltage is gently brought up from a very low or dead state, usually at a rate of 1/10 of the regular charge current;        2. next, the battery is charged at a constant current level, until the voltage across the battery reaches its design level (say, 4.2 VDC). At this point the battery will only be at 40 to 70% of full capacity; and then        3. charging of the battery continues at a constant voltage level (again, say 4.2 VDC) until it is fully charged. In this mode, the current drawn by the battery will drop over time. When the charge current has dropped to 10% of the initial charge rate, or some other limit determined by the battery manufacturer, charging is stopped.        
Charging must stop at this point because trickle charging is not acceptable for lithium batteries; an overcharge will damage the cells, possibly plating out lithium metal and becoming hazardous.
Hence, lithium batteries are almost invariably used with battery charge controllers designed to their particular charging parameters.
Unfortunately, most battery charge controllers are designed to draw from a high capacity power supply with a steady voltage that will not sag appreciably under its current demands. This is a problem when one attempts to use a power supply with limited capacity. Some computer data buses such as USB (universal serial bus) buses can be used to provide power to external devices, but while such power supplies are very convenient, they have limited capacity.
The majority of personal computers (PCs) and laptop computers available today, are provided with one or more USB ports as standard components. USB ports are designed to support data communication at speeds of 12 megabits and 1.5 megabits per second, support PnP (Plug and Play) installation software, and support hot plugging (i.e. devices can be connected and disconnected while the PC is running). Thus, USB ports are often used as interfaces to connect keyboards, mouses, game controllers, printers and scanners to PCs.
As well, USB ports are operable to supply limited power to connected external devices. The standard USB specification requires that “high-power” USB ports be operable to provide a supply voltage of 4.75-5.25 VDC and a supply current of at least 500 mA (often referred to as “five units”). The specification for “low-power” USB ports requires a supply voltage of 4.40-5.25 VDC and current of 100 mA (referred to as “one unit”).
USB ports would seem to be a very logical choice as a power supply for portable devices for a number of reasons. To begin with, USB ports supply a low DC voltage supply which is often very close to, or just above, the voltage of the battery being charged (many portable devices having battery voltages in the range of 2.5-4.5 VDC). As well, many portable devices may be operable to upload and download data or software, to and from a personal computer or a laptop computer (often referred to as “syncing”). Thus, many portable devices are supplied with docking cradles as shown in the system diagram of FIG. 1. This is quite a straightforward system, as the docking cradle 10 is connected to a USB port 12 of a personal computer (PC) 14, via a simple USB cable and connectors 16. The portable device 18 need only be set down in the docking cradle 10 and an electronic connection to the PC 14, is made.
If the USB port 12 has sufficient power, it makes much more sense to use the USB port 12 to supply charging power to the portable device 18, rather than using a separate AC charger. For example:    1. a USB power supply will have less electrical noise than an AC charger, unless the AC charger incorporates large DC capacitors or inductors;    2. an AC charger requires either a heavy transformer or an expensive switching power supply, neither of which would be required if USB power is used;    3. in the USB power supply implementation, the cable and connectors 16 used to connect the docking cradle 10 to the PC 14 could be used to carry both power and data, so no extra physical components would be required at all. In contrast, an AC power supply would have to be provided as a separate physical component from a USB data cable; and    4. there are no universal standards for AC power supplies; a given AC power supply may require 120 VAC or 240 VAC as an input, and may provide 3, 4.5, 6, 7.5 or 9 VDC out, with one of large number of different possible connectors and polarities. A traveler who forgets his AC power supply at home, may not be able to find a suitable replacement.
In contrast, the USB standard is widely accepted, so that a traveler whose mobile device is equipped with a USB connector will have a much greater chance of finding a charging source.
Thus, it would clearly be desirable to use USB power to charge portable devices. Unfortunately though, as noted above, USB ports can only provide limited power. The problem becomes clear when considering the block diagram of FIG. 2. In this scenario, the portable device 18 and the battery charge controller 20 are connected to the USB port 12 in parallel, as under charging conditions the control switch 22 will be toggled so that the portable device 18 draws power from the USB port 12. When the battery 24 has become fully charged by the battery charge controller 20 and the USB port 12 power removed, the control switch 22 is then toggled so that the portable device 18 draws power from the battery 24. This type of circuit may work in some circumstances, but it is not acceptable where the power source has limited capacity.
If one attempts to power the portable device 18 and battery charge controller 20 simultaneously from the USB card 12, it is quite likely that too great a load would be placed on the USB card 12. The excessive load on the USB card 12 may result in an undervoltage or low current condition that could result in a number of undesirable problems such as: the battery 24 not becoming properly charged or becoming permanently damaged, or the portable device 18 operating erratically or becoming damaged.
As an alternative, the battery 24 and portable device 18 could be arranged as shown in FIG. 3 so that they are both fed by the battery charge controller 20. While such a design would reduce the total power drawn by the portable device 18 and battery 24 in combination, there are a number of other problems:    1. most importantly, there is still no control over the total power that is drawn from the USB port 12;    2. the power drawn by the portable device 18 could disturb the carefully designed protection and charging mechanisms of the battery charge controller 20;    3. the portable device 18 and battery 24 compete arbitrarily for the available power, so they may adversely affect the operation of one another. If the available voltage drops too low, or insufficient current is available, either device may operate erratically, or fail all together;    4. if the battery 24 is in a deep-discharge state when power is turned on, the voltage to the portable device 18 will be dragged down to the level of the deep-discharged battery. Typically, portable devices 18 will not be operable at such a low voltage level; and    5. the current that must be supplied to the battery 24 and the portable device 18 must be dissipated by the battery charge controller 20 or an external semiconductor in some way. The more power that is dissipated, the larger the battery charge controller 20 (or the external semiconductor driven by the battery charge controller 20) must be. Generally, a semiconductor's ability to dissipate power varies with its surface area, thus, if the power dissipation is doubled, the semiconductor must increase by four times the surface area.
New, dedicated battery charge controllers could be developed which are designed to operate with a USB power supply and a portable device 18, but that would be an expensive and complicated solution. Each battery charge controller would have to be designed to suit a particular pairing of a portable device 18 and battery 24 because it would have to take the power consumption requirements of both components into account.
There is therefore a need for a method and apparatus which allows standard computer data busses such as USB ports to simultaneously power portable devices 18 and their associated battery charge circuits 20 without having to design new battery charge controllers with very specific applications. This design must be provided with consideration for the tight operating parameters of battery charging circuits, the limited physical board area in portable devices, and the reliability and complexity of the design.